Unit 3: Atomic Structure
Sections 1-2

warmup
All matter is composed of very small particles called atoms.
In middle school science you learned about the atom.
1.
Draw a picture of a typical atom. Identify and label the location of the nucleus, electrons, neutrons, protons.

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I can summarize Dalton's theories.
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I can describe properties of protons, neutrons and electrons, who discovered them and where they are found in an atom.

Section 1
What is an atom? Describe it’s size
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Definition: The atom is the smallest particle of an element that retains the characteristics of that element.
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A typical atom is 0.000000001 meters across -- that's one billionth of a meter
Video: Just how small is an atom 5:20

What is an atom?
A.
B.
C.
An atom is the smallest particle that can exist
An atom is the smallest particle that still has characteristics of an element
An atom is a solid sphere, like a marble only really really small

Section 1
Dalton’s 4 Postulates (1808)
1.
All elements are composed of tiny
indivisible particles called atoms
2.
Atoms of the same element are identical.
Atoms of different elements are different from another element.
3.
Atoms of different elements can chemically combine to form compounds
4.
Any chemical reaction is simply a re-arrangement of atoms, but the atoms John Dalton 1776-1844
“Father of Modern Chemistry” are not changed into different elements.

Section 2
Structure of the atom
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Electrons
Discovered by
J.J. Thomson in 1897.
Sir Joseph John Thomson (1856-
1940)
Nobel Prize for Physics (1906)
•Discovered the electron
•First to discover evidence for isotopes of stable elements.

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Section 2
Structure of the atom
Thomson created and used a Cathode Ray Tube to study the existence of electrons.
He passed an electric current through a sealed vacuum.
Negatively charged electrons are repelled by the negatively charge cathode, and are attracted to the positively charged anode causing a glowing cathode ray to appear
• Thomson - determined the presence of these electrons in all gases and concluded that electrons were part of
all atoms

Section 2
The Atomic Nucleus

In Thomson’s “Plum Pudding Model”: Scientists assumed the negative electrons were evenly
distributed throughout a positive atom.

Which of Dalton’s four theories did J.J.
Thomson disprove in his famous CRT experiment?
A.
B.
C.
D.
All elements are composed of tiny indivisible particles called atoms
Atoms of the same element are identical. Atoms of different elements are different from another element.
Atoms of different elements can chemically combine to form compounds
Any chemical reaction is simply a re-arrangement of atoms, but the atoms are not changed into different elements.

Section 2
Structure of the atom
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Robert Millikan - experimented to find the relative electrical charge of an electron to be –1
Millikan discovered electron mass to be 1/1840 of the mass of a hydrogen proton
Electrons in an atom determine its chemical properties
Robert Millikan 1869-1953
Nobel Prize for Physics 1923 first measured the electron charge and mass

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Section 2
Structure of the atom
Protons
Since atoms are electrically neutral, there
must be a particle that neutralizes the negative charge of an electron
Protons were discovered by Eugen
Goldstein in 1886 who observed additional rays in a cathode ray tube that traveled in the opposite direction of the cathode ray.
Protons have a electrical charge of +1 and a mass of 1 amu.
The number of protons in an atom determines the type of element
Eugen Goldstein
1850-1930 discoverer of the proton

Section 2
Structure of the atom
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Neutrons

James Chadwick discovered the existence of neutrons in 1932 using beryllium, alpha rays and known atomic masses.

Neutrons hold the protons together and thus contribute to the stability of the atomic nucleus.

Neutrons have a mass of 1 and no electric charge.
James Chadwick 1891-1974
Nobel Prize for Physics 1935

It has a very small mass, and a negative charge. What is it and where is it located in the atom?
A.
B.
C.
D.
A Neutron is located outside the nucleus
A Proton located in the nucleus
An Electron is located in the nucleus
An Electron is located in the electron cloud outside the nucleus.

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Section 2
Structure of the atom
In 1909 Ernest Rutherford disproves the plum pudding model by bombarding gold foil with alpha particles(+).
Ernest Rutherford
Nobel Prize for
Chemistry 1908

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Section 2
Structure of the atom
According to the Thomson’s popular “Plum Pudding” atomic theory of the time, the alpha particles should pass through the foil with only a slight deflection
Gold Atom
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Instead, Rutherford observed that almost all particles went straight thru, but a few of the particles were deflected at odd angles or sometimes directly backwards !!
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This result was completely unexpected.

Section 2
Rutherford’s planetary model
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Rutherford uses this information to propose a new atomic theory.
The atom is mostly made up of empty space.
The positive charge of the atom, and most of the mass, is concentrated in a small area
This small, dense area is known as the nucleus , and is surrounded by orbiting electrons.

Section 2
Rutherford’s Experiment a
= Alpha Particle
2 protons + 2 neutrons
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 http://highered.mcgraw-hill.com/sites/0072512644/student_view0/chapter2/animations_center.html#
Animations
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Cathode Ray Tube
Alpha Particle Scattering …Rutherford’s experiment

In his famous gold foil experiment, Rutherford shot alpha particles into gold.
What result did he prove?
A.
B.
C.
D.
Atoms have a large diffuse nucleus, like a pudding.
Atoms are made up of mostly empty space
Atoms have a small dense positively charged nucleus
The nucleus is actually much smaller and more dense than anyone knew

Atom Facts
Charge
Mass
Location in Atom
Discoverer
Function
Electron
-1
1/1840
(almost negligible) electron cloud
Proton
+1
1 amu nucleus
J.J. Thomson Goldstein determines chemical properties determines an elements identity
Neutron
0
1 amu nucleus
James
Chadwick holds nucleus together

Atom Facts
Charge
Mass
Location in Atom
Discoverer
Function
Electron
-1
1/1840
(almost negligible) electron cloud
Proton
+1
1 amu nucleus
J.J. Thomson Goldstein determines chemical properties determines an elements identity
Neutron
0
1 amu nucleus
James
Chadwick holds nucleus together

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To Summarize Dalton’s atomic theory
Describe size of an atom
Distinguish among protons, electrons and neutrons in terms of relative mass, charge, location and function
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Who Discovered…..
the electron?
the proton?
the neutron?
J.J. Thomson
Eugen Goldstein
James Chadwick

Distinguishing between atoms
Atomic Number
Section 3
Atomic Number

Atom Facts Review
Electron
Charge
Mass
Location in Atom
Discoverer
Function
-1
1/1840
(almost negligible) electron cloud
Proton
+1
1 amu nucleus
J.J. Thomson Goldstein determines chemical properties determines an elements identity
Neutron
0
1 amu nucleus
James
Chadwick holds nucleus together

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1.
2.
3.
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Section 3
Atomic number
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The atomic number of an atom is equal to the number of protons in an atom
It is also equal to the number of
electrons in a neutral atom
Why would the number of protons and electrons need to be the same in an atom?

Mass Number
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The mass of an atom is concentrated in the nucleus and is made up of protons and neutrons
An atom’s mass number is the total number of protons and neutrons in its nucleus.
i.e. Mass # = protons + neutrons
The number of neutrons in an atom can be determined by a quick calculation:
Number of neutrons = mass number – atomic number

What is the mass number of an atom of
Beryllium that has 5 neutrons?
A.
B.
C.
D.
8
9
4
5
Mass # = protons + neutrons

An atom of Titanium has 22 protons and has a mass number of 48. How many neutrons are in this atom?
A.
B.
C.
D.
22
25
26
70
Mass # = protons + neutrons

The composition of an atom is given in the following shorthand:
7
15
N
Protons = ______
Neutrons = ______
Electrons = ______

A.
B.
C.
D.
What element is this?
calcium chlorine carbon carbonite
14
C
6

What is the atomic number?
14
C
6

What is the mass number?
14
C
6

How many neutrons in this atom?
14
C
6

Isotopes
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Remember! Neutrons in an atom hold the nucleus together and make it stable.
An isotope of an element is an atom of that element with the normal number of protons, but a different number of
neutrons.
Isotopes are chemically alike because they have the same number of protons and electrons, but differ slightly from Dalton’s atomic theory since the number of neutrons may differ.
Isotopes are also named using the element’s name followed by the number of protons + neutrons.
example: Carbon-14

Isotopes
Isotopes of Carbon
Protons Electrons Neutrons Name
6
6
6
6
6
6
7
8
9
Carbon-13
Carbon-14
Carbon-15
Name
13
C
6
14
C
6
15
C
6

Isotopes
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Some highly unstable isotopes can be useful:
Carbon – 14:
 half life: 5730 years archaeological carbon dating
Iodine – 131:
 half life: 8.02 days treatment of thyroid disorders
Americium – 241: half life: 431.2 years
 smoke detectors

Which of Dalton’s four theories was disproven by the discovery that elements have isotopes with differing neutrons?
A.
B.
C.
D.
All elements are composed of tiny indivisible particles called atoms
Atoms of the same element are identical.
Atoms of different elements are different from another element.
Atoms of different elements can chemically combine to form compounds
Any chemical reaction is simply a re-arrangement of atoms, but the atoms are not changed into different elements.

How to calculate Atomic Mass
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Most elements have more than one stable isotope as you find them in nature.
Definition: Atomic Mass the weighted average of the masses of the isotopes of an element.
Some isotopes occur more often than others so the atomic mass of an element is the average of the isotopes’ mass weighted by the
abundance of that isotope.
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To calculate an element’s atomic mass, you need to know the number of stable isotopes that exist for that element, the mass of each isotope, and the percent abundance of that isotope.

How to calculate Atomic Mass
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Example: in any sample of the element neon that you find, there are three isotopes present.
Most of it, 90.48%, is neon-20 19.992 amu
0.27% is neon-21 20.993 amu
9.25% is neon-22 21.991 amu
To calculate the average atomic mass multiply the mass of each isotope by its %abundance and add them together.
Neon-20:
Neon-21:
Neon-22:
19.992 x .9048
20.993 x .0027
21.991 x .0925
=
=
=
18.089
0.057
2.034
Atomic mass of neon is20.180 amu
Try it this way on your calculator:
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19.992 x .9048 + 20.993 x .0027 + 21.991 x .0925 [enter]

Bromine has two naturally occurring isotopes. Calculate the atomic mass.
35
79
35
81
Br = 78.92 amu (50.69% abundant)
Br = 80.92 amu (49.31% abundant)
Round to 2 decimal places,
Enter numeric answer use @ key for the
. decimal point

Section 4 the Periodic Table
How it came about?
Location, location, location!!!

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Warm up Questions
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1.
2.
3.
An atom has an atomic number of 4 and a mass number of 10
How many protons, electrons and neutrons does this atom have?
What element is this atom?
How would you write the name of this isotope?
1.
2.
An atom of aluminum has a mass number of 27.
How many protons, electrons and neutrons does this atom have
How would you write the name of this isotope of aluminum?

Section 4 – Development of the periodic table
Dmitri Mendeleev
A. Dmitri Mendeleev

Mendeleev was the first scientist who tried to organize the elements into a logical pattern.
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In 1869, he first listed elements in columns in order of increasing atomic mass.
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That arrangement didn’t show any logical trends, so he arranged columns with elements that have similar properties side by side in a table. (p123)
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Using this table, he actually predicted the properties of elements yet to be discovered!!!!
Dmitri Mendeleev
(1834-1907)

Section 4 – Development of the periodic table
Henry Moseley

Henry Moseley then arranged the elements according to atomic number, keeping the conditions Mendeleev started with.
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Moseley’s arrangement allows us to predict the physical and chemical properties of elements simply based on their location in the table.
Henry Moseley
1887-1915

B.
The Modern Periodic Table
-- The Periods--
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Mendeleev’s and Mosley’s discoveries give rise to
Periodic Law

Horizontal rows are called periods. There are a total of 7 periods.
Periodic law: when the elements are arranged in order of atomic number, there is a repetition of physical and chemical properties.
Periods go across …..
Periods go across …..

B.
The Modern Periodic Table
--- the groups -----
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Vertical columns are called groups.
The elements in any group tend to have the same physical and chemicals properties.

There are three general classes of elements:
1.
Metals solid at room temp., conduct electricity, ductile, malleable
2.
Nonmetals usually non-lustrous, poor conductors of electricity, gases
3.
Metalloids intermediate properties between metals and nonmetals

Stair Case divides metals from non-metals
Metals
Metalloids
Non-Metals

Significant groups
Alkali metals
Alkaline earth metals
Transition Metals
Halogens
Nobel Gases

Group 1A:
Alkali metals = H, Li, Na, K, Rb, Cs, Fr react violently with water

Group 2A:
Alkaline earth metals =
Be, Mg, Ca, Sr, Ba, Ra
Lustrous, Silvery, somewhat reactive

Transition Metals (Group B) all have similar metal properties: solid at room temp., conduct electricity, ductile, malleable

Group 7A:
Halogens = F, Cl, Br, I, At are all nonmetals and are very reactive

Group 0 also called Group 8A
Noble gases = He, Ne, Ar, Kr, Xe, Rn are all nonmetals and also known as inert gases because they have
virtually no chemical reactivity.

Quiz yourself!
Match the element to its group
Groups
1. ____ Cesium A. halogen
2. ____ Fe
3. ____ Sr
4. ____ Neon
5.___ Br
B. noble gas
C. alkali metal
D. transition metal
Match the property to its group
6. ___ no reactivity
7. ___ silvery, somewhat reactive
8. ___ all have similar metal properties
9. ___ very reactive non-metals
E. alkaline earth metal 10. ___ react violently with water

Quiz yourself! answers!
Match the element to its group
Groups
1. _ C __ Cesium A. halogen
2. _ D __ Fe
3. _ E __ Sr
4. _ B __ Neon
5._
A _ Br
B. noble gas
C. alkali metal
D. transition metal
Match the property to its group
6. _ B _ no reactivity
7. _ E _ silvery, somewhat reactive
8. _ D _ all have similar metal properties
9. _ A _ very reactive non-metals
E. alkaline earth metal 10. _ C _ react violently with water